中心扩散法
var longestPalindrome = function(s) {
let len = s.length;
if(len == 0 || len == 1){
return s
}
let start = 0;
let maxlen = 1;
for(let i = 0; i<len;i++){
let len1 = expendCenter(s,i,i);
let len2 = expendCenter(s,i,i+1);
let curmaxlen = Math.max(len1,len2);
if(curmaxlen>maxlen){
maxlen = curmaxlen;
start = Math.ceil(i - (maxlen-1)/2);
}
}
return s.slice(start,start + maxlen);
};
var expendCenter = function(s,l,r){
let L = l;
let R = r;
while(L>=0 && R<s.length && s[L]==s[R]){
L--;
R++;
}
return R-L-1
}